1. Technical Field
The present invention is directed toward the field of optical communication circuits using laser diodes. More specifically, the invention provides a linear laser driver circuit that is particularly well-suited for use in driving a laser diode for use with an optical communication system. As part of such a system, the laser diode is pulsed on and off at a very high frequency in order to communicate pulses of light over an optical fiber.
2. Description of the Related Art
Laser diodes and their associated driving circuitry are known in this field. Laser diodes are characterized by a temperature dependent threshold current ITH, above which point the diode begins to act like a laser. FIG. 1, for example, is a plot 10 showing the typical light output (P) 12 v. current (I) 14 characteristic for a laser diode at two operating temperatures 16, 18. As seen in this plot 10, the laser diode threshold current ITH is lower ITH1 at the lower temperature than at the higher temperature, where the threshold current is ITH2. The Quantum Efficiency (QE) of the diode is characterized by the slope of the P v. I curve.
Simple laser driver circuits turn the laser diode on and off for each pulse of light to be transmitted over the fiber. FIG. 2 is a plot 20 showing a plurality of light pulses output from such a laser driver circuit. The y-axis in this plot shows light output (P) 22, and the x-axis shows time (t) 24. As seen in this plot 20, the problem with this type of simple on/off driver circuit is that it causes the laser diode to cross over the laser threshold current level (ITH), which causes a ringing phenomenon 26 to occur on the output pulse that consists of a plurality of light spikes. These light spikes are caused by the laser transitioning from operating like a light emitting diode to operating like a laser. After a short burst, these spikes 26 subside, and the output pulse is relatively flat 28, until the pulse terminates.
In order to cure the problem shown in FIG. 2, more complex laser driver circuits have been used in this field that typically include two feedback loops. The first loop regulates the laser diode""s average light output and maintains the laser above the threshold current level (ITH) even during off periods. This eliminates the ringing phenomenon shown in FIG. 2 since the diode is always above the threshold current. The second feedback loop is used to regulate the modulation index, and requires a complex analog gain control stage to adjust the laser diode""s extinction ratio ER. Often, these feedback circuits require temperature compensation thermistors and multiple factory adjustments to control the extinction ratio.
A linear laser diode driver circuit is provided in which a solid state laser diode and its back-facet photodiode are configured into the feedback loop of a high-speed operational amplifier. In this configuration, the light output from the laser diode is directly proportional to the input voltage to the operational amplifier and is independent of the laser diode""s temperature characteristics.
The linear laser driver circuit has the advantage that it can transmit optical pulses that are bandwidth limited and spectrally shaped. In one embodiment of the invention, the linear laser driver can be coupled to a spectral-shaping low pass filter, such that the input pulse train can be first sent through the spectral-shaping low pass filter in order to limit the bandwidth before driving the laser. This filtering allows much greater control of spectral components in the output signal (and hence much greater control over noise) than can be accomplished using external wave shaping components such as ferrite beads. With such a low pass filter, the spurious frequency components can be filtered by any arbitrary amount (such as 50 dB below the fundamental components), in order to spectrally shape the input signal to the linear laser driver.
One aspect of the invention provides a laser diode communication circuit, comprising: (1) an input pulse communication signal; (2) a laser diode having a back-facet photodiode for monitoring the output light signal from the laser diode; (3) a current source coupled to the laser diode; and (4) a first operational amplifier having an input coupled to the input pulse communication signal and an output coupled to the current source for causing the current source to supply current to the laser diode in relation to the input pulse communication signal, wherein the laser diode and its back-facet photodiode are configured in a feedback loop between the output and the input of the first operational amplifier.
Still another aspect of the invention provides a laser diode communication circuit, comprising: (1) a laser diode having a back-facet photodiode for monitoring the light output from the laser diode; (2) a fiber optical cable coupled to the laser diode for communicating light output signals from the laser diode; (3) an input pulse communication signal; and (4) a linear laser diode driver circuit having an input node, a current drive output node, and a feedback node, wherein the input node of the linear driver circuit is coupled to the input pulse communication signal, the current drive output node is coupled to the laser diode for driving the laser with a particular current level, and the feedback node is coupled to the back-facet photodiode of the laser diode, such that the laser diode is configured in a feedback loop between the current drive output node and the feedback node of the linear driver circuit.
These are just a few of the many aspects of the invention, which are detailed below in reference to the attached drawing figure. Other aspects and variations thereof will be apparent to one of ordinary skill in this field upon reviewing this application.
The present invention provides many advantages, such as: (1) eliminates the need for thermisters and extinction ratio factory adjustments; (2) low cost; (3) easier to manufacture; (4) automatic compensation of laser variation over temperature and time; (5) simpler circuitry; (6) provides linear light output versus control voltage; (7) provides precision control of light on/off levels; (8) eliminates spurious laser emissions; and (9) eliminates the need for external wave-shaping components.
These are just a few of the many advantages of the present invention, which is described in more detail below in terms of the preferred embodiments. Not all of these advantages are required to practice the invention, and this listing is provided simply to illustrate the numerous advances provided by the invention. As will be appreciated, the invention is capable of other and different embodiments, and its several details are capable of modifications in various respects, all without departing from the spirit of the invention. Accordingly, the drawings and description of the preferred embodiments set forth below are to be regarded as illustrative in nature and not restrictive.